These studies were undertaken to investigate the effects of exposure to 2,3,7,8,-Tetrachlorodibenzofuran (TCDF) on immunocompetence in guinea pigs. Guinea pigs were orally given 6 doses of either 1.0, 0.5, 0.17 or 0.05 microgram of TCDF/Kg body weight in corn oil weekly over a 6 week period. Controls received corn oil alone. These studies indicated that cell mediated immune functions were suppressed at the higher dose levels, particularly at 1.0 microgram/Kg dose.